Various formation processes such as sputtering techniques or physical evaporation techniques are used for forming superconductive strips on substrates. However, the deposited superconductive material is usually covered with a certain surface film which tends to suppress the superconductivity. The superconductive strip is then subjected to a heat treatment in an oxidation ambient between about 800 degrees and about 950 degrees in centigrade for about an hour, and the heat treatment aims at refining the undesirable surface film.
However, a problem is encountered in the prior art refining technique in that cracks tend to take place in the superconductive strips due to thermal stress. This is because of the fact that the superconductive strips are usually different in thermal expansion coefficient from the substrate. The cracks deteriorate the production yield of the superconductive strips.